Bentonite for binding impurities during paper production

ABSTRACT

A method for binding impurities in paper production, comprising the following steps: 
     a) provision of a bentonite, the proportion of the monovalent cations, based on the cation exchange capacity (CEC) of the bentonite, being at least 0.7 and the CEC being more than 85 meq/100 g, preferably more than 90 meq/100 g, in particular more than 95 meq/100 g; 
     b) addition of the bentonite according to a) to a paper pulp or fiber suspension; 
     c) binding of the impurities to the bentonite in the pulp or fiber suspension 
     is described.

The present invention relates to the use of special bentonites having ahigh cation exchange capacity in the binding or removal of impurities inpaper production.

The removal or binding of impurities in paper production is becomingincreasingly important. The problem is also based on the fact that thewater occurring in paper production is circulated, impurities graduallybecoming more concentrated therein. These impurities can thus lead to avery wide range of product faults, such as, for example, to theformation of deposits on the rolls of the paper machine, to blocking ofthe wires by adhesion, etc. These effects lead to interruptions to paperproduction. In order to minimize the number of production stops, it isdesirable to bind the impurities occurring in the circulation water byusing polymers or adsorbents in the headbox itself. Most relevantimpurities are negatively charged. These are, for example, humic acids,tree resin colloids, lignin derivatives, ligninsulfonates, which areintroduced from the fibers into the paper circulation. There are alsoanionic impurities, which are introduced into the paper machine byrecycling of broke. This broke is typically re-dispersed and introducedinto the paper machine. As a result, the ingredients and auxiliariespresent therein are completely recycled into the circulation. Forexample, carboxymethylcelluloses, polyacrylates, polyphosphonates andsilicates are additionally introduced thereby. Further anionic chargedimpurities are the latices which are used in the paper coat, which aretypically hydrophobic but also carry anionic charges. These have astrong tendency to agglomeration, the agglomerates being deposited astacky, white residues on the paper machine (so-called white pitch).

The prior art extensively describes the discharge of stickies by the useof talc. Thus, according to P. Biza, E. Gaksch and P. Kaiser“Verbesserter Austrag von Stickys durch den Einsatz von Talkum [Improveddischarge of stickies by the use of talc]”, Wochenblatt fürPapierfabrikation 11/12 (2002), page 759 et seq., the effect of talc onthe reduction of tacky deposits has been documented since the beginningof the last century at the latest. Almost all known natural andsynthetic tacky substances are hydrophobic. Talc is very suitable forbinding these stickies because it has a naturally hydrophobic surfacewhich enables it to be readily adsorbed onto sticky surfaces and to makethem less tacky by coating.

Furthermore, the use of montmorillonites, such as bentonite, forcontrolling impurities in the paper pulp is described, for example, inU.S. Pat. No. 5,368,692. The alkali treatment of bentonites is alsodiscussed as a possibility.

U.S. Pat. No. 4,964,955 likewise describes a process for reducing theimpurities in paper production. There, a particulate compositioncontaining (a) a water-soluble cationic polymer which is applied to (b)a substantially water-insoluble particulate substrate is used forbinding impurities. The polymer should be sufficiently electropositiveso that the particulate composition has a zeta potential of at leastabout +30 mV. The polymer is preferably a poly(dialkyldiallylammoniumhalide). The substrate is, for example, a phyllosilicate mineral.

In a similar manner, EP 0 760 406 A2 relates to a combination of apoly(dadmac/acrylamide) and a bentonite for binding impurities.

GB 2 297 334 A in turn discloses the use of a smectic clay forcontrolling impurities, the smectic clay being modified as follows:monovalent exchangeable cations are present in an equivalent ionicfraction in the range of from 0.20 to 0.60; a first type of bivalentexchangeable cations is present in an equivalent ionic fraction in therange of from 0.40 to 0.80; and a second type of bivalent exchangeablecations is present in an equivalent ionic fraction in the range of from0.00 to 0.20, the first type of bivalent exchangeable cations comprisingcalcium and the second type of bivalent exchangeable cations comprisingmagnesium.

Many of the compositions used in the prior art for binding impuritiesare very expensive and not optimally suitable for certain impuritycompositions. There is therefore a constant need for compositions forbinding impurities in paper production.

An object of the present invention was therefore to provide an improvedprocess for binding impurities in paper production, in which acomposition which is easy and economical to prepare can be used andwhich permits a high degree of binding of impurities, includinghydrophobic fractions.

According to one aspect of the invention, this object is achieved by themethod as claimed in claim 1.

Thus, in the present invention, it was surprisingly found thatsurprisingly good binding of impurities in a method for bindingimpurities in paper production can be provided by the use of a bentonitewhich has a proportion of the monovalent cations, based on the cationexchange capacity (referred to herein as CEC), of at least about 0.7(i.e. 70%), and a CEC (total) of at least 85 meq/100 g.

In the context of the present invention, impurities are understood asmeaning both tacky substances, referred to in the literature asstickies, and so-called pitch, i.e. primarily tree resin components.Reference may be made here to the statements made in the introduction tothe description with regard to the impurities. A detailed list of thepitch and stickies constituents is to be found, for example, inWo01/71092 on pages 1 and 2, and the disclosure there is herebyexpressly incorporated by reference into the present description.

As mentioned above, the impurities are thus primarily anionic(negatively charged) or hydrophobic. Thus, it was all the moresurprising that the highly activated bentonites used according to theinvention and having a high CEC can very readily bind both anionic andhydrophobic impurity fractions and can neutralize them in their harmfuleffects. The bentonites used according to the invention themselves havea relatively high negative layer charge and subsequently make this high(negative) surface charge available in delaminated form to the paperpulp. Thus, good binding of impurities would not be expected for anionicor hydrophobic impurities. It would also be expected that a calciumbentonite binds such impurities better because a major part of thecharges of the bentonites is saturated by the calcium ions and thesecould immobilize impurities, for example, via soap formation and fattyacids in the tree resin. In particular, the stickies, such as tree resinparticles, contain many rather nonpolar (hydrophobic) components, e.g.triglycerides. These should bind particularly well to nonpolar surfaces,such as, for example, those of talc. Talc has no surface charges and istherefore also described in the prior art as being optimum for thebinding of (hydrophobic) impurities.

The results in the context of the present invention, according to whichboth nonpolar and anionic impurities can be efficiently bound in themethod according to the invention with bentonites which make available alarge surface having numerous negative charges are therefore unexpected.

The method according to the invention with the use of the specialbentonite described herein can be used generally in all methods forpaper or cardboard production. Accordingly, the expressions paper pulpand fiber suspension are intended generally to include allimpurity-containing compositions or streams which are used in paperproduction. Otherwise, the expressions “pulp” and “fiber suspension” arefamiliar to the person skilled in the art and need not be explained indetail here.

In a preferred embodiment according to the invention, the pulp or thefiber suspension is a (fine) groundwood-containing suspension.Groundwood is in general finely digested (finely beaten wood, generallywithout further chemical or thermal treatment). The groundwoodsuspension is either used directly after comminution or is subjected toa peroxide bleach, in which case so-called peroxide-bleached groundwoodforms. It has surprisingly been found that the bentonite used accordingto the invention gives particularly good results in the case of papertypes containing groundwood or peroxide-bleached groundwood. However,the method according to the invention can also be advantageously used inthe case of other paper types. Thus, for example, the pulp or fibersuspension (in addition to the groundwood) may also contain highlypurified fiber fractions, as is the case, for example, in so-callednewsprint paper. The invention furthermore gives very good results inthe case of so-called “deinked pulp” (DIP). This is a paper stock whichis produced from wastepaper. In particular, hydrophobic stickies occurthere, from the stickies of magazines and newspapers. These too can bereadily bound in the end product by the bentonite used according to theinvention. Further so-called paper stocks in which the bentoniteaccording to the invention can be advantageously used comprise TMP(thermomechanical pulp) sulfate pulp, sulfite pulp and mixtures ofdifferent chemical pulps. Depending on the paper type and localizationof the paper mill, such chemical pulps are mixed in different ratios andadapted to the material requirements of the end product.

In an advantageous embodiment according to the invention, the preferredgroundwood fraction in the paper pulp or fiber suspension is at least10% by weight, in particular at least 30% by weight, based in each caseon the dry weight of the total pulp or suspension.

The bentonite in the method according to the invention probably actswithout the invention being limited to the correctness of thisassumption, in that it binds the impurities or interacts with them andthus, counteracts the aggregation and deposition on the parts of thepaper machine, such as, for example, the rolls.

According to the invention it is important that the bentonite used has acation exchange capacity (CEC) of at least 85 meq/100 g, preferably atleast 90 meq/100 g, in particular at least 95 meq/100 g.

“Cation exchange capacity” (CEC) is understood as meaning the sum of allexchangeable cations, stated in meq/100 g and determined by the CECanalysis method as explained below before the example section(determination of the cation exchange capacity). The cation exchangecapacity thus comprises, for example, the sum of all exchangeabledivalent and monovalent cations, such as calcium, magnesium, sodium,lithium and potassium ions. For the determination of the cation exchangecapacity, the bentonite is treated with an ammonium chloride solution.Owing to the high affinity of the ammonium ions for the bentonite,virtually all exchangeable cations are exchanged for ammonium ions.After separation and washing, the nitrogen content of the bentonite isdetermined and the content of ammonium ions is calculated therefrom.

It is possible to use both natural bentonites and bentonites obtained byactivation, for example from calcium bentonites, provided that the aboveconditions for the fraction of monovalent cations, based on the CEC, andthe minimum values for the CEC are complied with. Processes forproducing or activating bentonite are known per se to the person skilledin the art and need not be explained in detail here. For example, it ispossible to start from a calcium bentonite having a suitable CEC and totreat it with an alkali metal carbonate, e.g. sodium carbonate. In thetreatment or activation of the phyllosilicate, contact can beestablished in any desired manner familiar to the person skilled in theart, for example by preparing a solids mixture, a suspension with thephyllosilicate and the sodium carbonate or treatment or spraying of thephyllosilicate with a solution of the sodium carbonate.

For example, according to the first method variant, a calcium-containingcrude bentonite having a water content of from about 25 to 40% by weightis kneaded with solid sodium carbonate, dried and milled. The crudebentonite is broken beforehand into pieces of less than 3 cm diameter.If the crude bentonite does not have the stated water content, this isestablished by spraying with water.

The activation can also be effected, for example, as follows: 350 g ofcrude bentonite having a water content of from about 30 to 35% by weightare introduced into a mixing apparatus (for example a Werner &Pfleiderer mixer (kneader)) and kneaded for 1 minute. The amount ofsodium carbonate (soda) which corresponds to the difference between CECand sodium content of the bentonite is then added while the mixingapparatus continues to run and further kneading is effected for 10 min.Here, the added amounts are based on the anhydrous bentonite. Ifrequired a little more distilled water is added so that the kneadedmaterial “shears” thoroughly. The kneaded material is then comminuted into small pieces and dried to a water content of 10±2% in aforced-circulation drying oven at about 75° C. for from 2 to 4 hours.The dry material is then milled in a rotor beater mill (e.g. in a Retschmill) over a 0.12 mm sieve. The CEC and the fraction thereof of sodiumions were determined as described further below.

Overactivation of the bentonite, for example with soda, is likewisepossible, it being possible to use more soda than would bestoichiometrically required for complete activation of the bentonite.

In a particularly preferred embodiment according to the invention, thestated fraction of monovalent cations is based on the fraction ofsodium, potassium and lithium ions, in particular of sodium ions.

In a preferred embodiment according to the invention, the bentonite usedhas a swellability of at least 25 ml/2 9, in particular of at least 30ml/2 g, more preferably at least 35 ml/2 g. Thus, it has surprisinglybeen found that bentonites having such high swellability permitparticularly advantageous binding of impurities. The swelling volume isdetermined as follows: a calibrated 100 ml measuring cylinder is filledwith 100 ml of distilled water. 2.0 g of the substance to be measuredare introduced slowly in portions of from 0.1 to 0.2 g onto the watersurface. After the material has sunk, the next portion is added. Afterthe end of the addition, a waiting time of 1 hour is allowed and thevolume of the swollen substance is then read in ml/2 g.

Furthermore, it has been found that the proportion of iron ions, basedon the CEC, should preferably be less than about 0.005 (0.5%). It hasbeen found that such bentonites give better results with regard to thewhiteness of the paper pulp.

According to a further preferred aspect, the proportion of themonovalent cations, based on the CEC of the bentonite, is more than 0.7,in particular more than 0.8, preferably more than 0.81, more preferablymore than 0.85. It is furthermore preferable if the proportion ofcalcium and/or magnesium ions, based on the CEC of the bentonite, isless than 0.2, in particular less than 0.18, preferably less than 0.15.

In a further preferred embodiment according to the invention, the BETsurface area (determined according to DIN 66131) of the bentonites usedis less than 100 m²/g, in particular less than 90 m²/g. It is surprisingthat bentonites having a relatively low specific BET surface areaexhibit particularly advantageous binding of impurities in comparisonwith bentonites which can provide a higher specific surface area foradsorption of impurities.

Typically, the demand for cationic charges in the headbox decreaseswhile the method according to the invention is being carried out. Thisis demonstrated by the binding of the negatively charged impurities bycharge interactions.

The concentration of the impurities in paper production is typicallydetermined in the white water by the three customary processes of cationdemand (cationic charge demand), stability measurement and chemicaloxygen demand. In the case of cation demand, it is assumed that theimpurities are all negatively charged and the white water filters inshort-chain cationic polyelectrolytes. The consumption is converted intothe so-called cation demand. In the turbidity measurement, it is assumedthat the impurities are partly present in colloidal form and theirconcentration can be determined via the extinction caused by theturbidity. In the case of the chemical oxygen demand, the proportion oforganic compounds present is tested by means of an oxidizing agent.Although these methods are very widely used in the paper world, morerecent investigations have shown that they average over the totalingredients in white water and only partly detect particularly criticalimpurities. This arises, for example, from the fact that the so-calledtree resin colloids, some of which are composed of hydrophobiccompounds, may carry only small surface charges and hence contributelittle to the cation demand. On the other hand, lignins have a highcationic demand; if they are present in the white water, they interfereonly very little in paper production. More recent investigationsfurthermore show that the correlation between the turbidity measurementand the concentration of colloidal impurities is not always present.Owing to this more recent experience with the customary impuritydetermination methods, the additives according to the invention werealso characterized in their action by more recent methods. These are,for example, a gas chromatographic analysis of the white water by themethod of F. Orsa and B. Holmbom “A Convenient Method for theDetermination of Wood Extractives in Papermaking Process Waters andEffluents”, Journal of Pulp and Paper Science, Vol. 20 No. 12 December1994, pp J361. In the production of a groundwood-containing paper, theindividual tree resin components are determined in their concentrationby a gas chromatographic method. This is a complete, quantitativeanalysis, whereas the standard methods of determination, such asturbidity, cation demand and chemical oxygen demand, are actually to beconsidered as only being semi-quantitative at best. Furthermore, L.Vähäsalo et al. (loc. cit., cf. “Flowcytometrische Analyse desSiebwassers [Flow cytometric analysis of white water]”, further below,show that so-called flow cytometry is very suitable for determining thenumber of colloidal impurities in paper white waters. This new methodwas therefore also used in the present invention in order to show theimpurity-reducing effect of the bentonites according to the invention.

The addition of the bentonite used according to the invention to thepulp or fiber suspension can be effected at any desired point in thepaper production which is suitable for the person skilled in the art. Inparticular, the addition directly in the pulper is also advisablebecause a long contact time with the paper stock is possible there andthere is the probability of a high degree of binding of impurities.Further addition points are in the entire so-called high-consistencystock region. Addition for dissolved air-floatation for waterpurification is also conceivable. In many cases, an already presentaddition point for additives, for example in the form of a meteringapparatus or metering pump, will also be present in the apparatuses usedin each case for paper production, which apparatus or pump can be usedfor the addition of the bentonite used according to the invention. Thebentonite can be used both in powder form and in the form of asuspension or slurry. The suspension or slurry will in many cases permitbetter meterability and is more easily automatable in industrial,continuous processes.

It has furthermore been found that the effect of the bentonite usedaccording to the invention is particularly positive if a certainparticle size is maintained. Thus, according to a particularly preferredembodiment of the invention, the particle size of the bentonite ischosen so that in the wet sieve residue less than 2% by weight,preferably less than 1% by weight, in particular less than 0.5% byweight, is 45 μm. The determination of the wet sieve residue isexplained in more detail before the examples. The preferred particlesize can also be determined by the light scattering method (Malvern). Ina particularly preferred embodiment according to the invention, themedian particle size (D50) (based on the sample volume) is from 0.5 to10 μm, in particular from 2 to 6 μm, particularly preferably from 3 to 5μm.

In the present invention, it was also surprisingly found that the use ofthe bentonite used according to the invention leads to particularly goodbinding of impurities if talc is not used in the method. The use ofcationic polymers, such as, for example, poly(dadmac) or polyacrylamide,according to the prior art can also be reduced or even completelyomitted with the aid of the bentonite used according to the invention.

The amounts of bentonite used in the method according to the inventioncan be determined by the person skilled in the art in a routine mannerusing empirical experiments. In most cases, it is advantageous to useamounts of from 0.5 to 12 kg/t of paper pulp or fiber suspension,preferably from 1 to 8 kg/t, and in particular from 1.5 to 7 kg/t, basedin each case on the anhydrous pulp/suspension (dry weight).

In the present invention, it was surprisingly also found that the methodaccording to the invention permits not only very good binding of anionicimpurity fractions, such as fatty acids, but also outstanding binding orelimination of hydrophobic impurity fractions, such as sterols, sterylesters and triglycerides. The results achieved here surprisingly surpassboth those which were obtained with conventional bentonites and those oftalc.

A further aspect of the present invention relates to the use of abentonite as described herein for binding impurities in paperproduction. As mentioned above, the bentonite is preferably used in apaper pulp or fiber suspension which contains groundwood fractions.However, all paper types or pulps are covered by the use according tothe invention. The paper types mentioned further above, such as papertypes containing groundwood or peroxide-treated groundwood, those which(in addition to the groundwood) also contain highly purified fiberfractions, as is the case, for example, in so-called newsprint paper,so-called deinked pulp (DIP), TMP (thermomechanical pulp), sulfate pulp,sulfite pulp and mixtures of different chemical pulps are particularlypreferred.

Method section: Unless stated otherwise, the analytical methods statedbelow are used:

1. Determination of the Cation Exchange Capacity (CEC Analysis) and ofthe Cation Fractions

Principle: The clay is treated with a large excess of aqueous NH₄Clsolution and washed out, and the amount of NH₄ ⁺ remaining on the clayis determined according to Kjeldahl.

Me⁺(clay)⁻+NH₄ ⁺—NH₄ ⁺(clay)⁻+Me⁺

(Me⁺═H⁺, K⁺, Na⁺, ½Ca²⁺, ½Mg²⁺. . . )

Apparatuses: Sieve, 63 μm; conical flask with ground glass joint, 300ml; analytical balance; membrane suction filter, 400 ml; cellulosenitrate filter, 0.15 μm (from Sartorius); drying oven; reflux condenser;hotplate; distillation unit, VAPODEST-5 (from Gerhardt, No. 6550);graduated flask, 250 ml; flame AAS.

Chemicals: 2N NH₄Cl solution, Nessler's reagent (from Merck, Art. No.9028); boric acid solution, 2% strength; sodium hydroxide solution, 32%strength; 0.1 N hydrochloric acid; NaCl solution, 0.1% strength; KClsolution, 0.1% strength.

Procedure: 5 g of clay are sieved through a 63 μm sieve and dried at110° C. Thereafter, exactly 2 g are weighed into the conical flaskhaving a ground glass joint on the analytical balance by differentialweighing, and 100 ml of 2N NH₄Cl solution are added. The suspension isboiled under reflux for one hour. In the case of bentonites having ahigh CaCo₃ content, ammonia may be evolved. In these cases, NH₄Clsolution must be added until the odor of ammonia is no longerperceptible. An additional check can be carried out with a moistindicator paper. After a standing time of about 16 h, the NH₄⁺-bentonite is filtered off over a membrane suction filter and washedwith demineralized water (about 800 ml) until substantially free ofions. The testing of the wash water for freedom from ions is carried outfor NH₄ ⁺ ions with Nessler's reagent which is sensitive to them. Thewashing time can vary from 30 minutes to 3 days, depending on the typeof clay. The washed-out NH₄ ⁺-bentonite is removed from the filter,dried at 110° C. for 2 h, milled, sieved (63 μm sieve) and dried againat 110° C. for 2 h. Thereafter, the NH₄ ⁺ content of the bentonite isdetermined according to Kjeldahl.

Calculation of the CEC: the CEC of the clay is the NH₄ ⁺ content of theNH₄ ⁺ bentonite, determined by means of Kjeldahl (for CEC of some clayminerals, cf. appendix). The data are given in meq/100 g of clay.

Example: Nitrogen content=0.93%;

Molecular weight: N=14.0067 g/mol

${C\; E\; C} = {\frac{0.93 \times 1000}{14.0067} = {66.4\mspace{14mu} {meq}\text{/}100\mspace{14mu} g}}$

CEC=66.4 meq/100 g of NH₄ ⁺-bentonite

Exchanged Cations and the Proportions Thereof:

The cations liberated by the exchange are present in the wash water(filtrate). The proportion and the type of the monovalent cations(“exchanged cations”) are determined spectroscopically in the filtrateaccording to DIN 38406, part 22. For example, for the AAS determination,the wash water (filtrate) is concentrated, transferred to a 250 mlgraduated flask and made up to the mark with demineralized water.Suitable measuring conditions for FAAS are shown in the followingtables.

Element Calcium Potassium Lithium Magnesium Sodium Wavelength 422.7766.5 670.8 285.2 589.0 (nm) (202.6) Gap width 0.2 0.5 0.5 0.5 0.2 (nm):Integration 3 3 3 3   3 time (sec): Flame gasses: N₂O/C₂H₂ Air/C₂H₂Air/C₂H₂ N₂O/C₂H₂ Air/C₂H₂ Background no no no yes no comp.: Measuringconc. conc. conc. conc. conc. method: Ionization 0.1% KCI 0.1% NaCl 0.1%NaCl 0.1% KCI 0.1% KCI buffer: Burner 15-20° — — — — positionCalibration 1-5 mg/l 1-5 mg/l 2-10 mg/l 0.5-3 mg/l 1-5 mg/l standard(5-40 mg/l) (mg/l):

Element Aluminum Iron Wavelength 309.3 248.3 (nm): Gap width 0.5 0.2(nm): Integration time (sec): 3 3 Flame gasses: N₂O/C₂H₂ Air/C₂H₂Background comp.: yes no Measuring method: conc. conc. Ionizationbuffer: 0.1% KCl — Burner position — — Calibration 10-50 mg/l 1-5 mg/lstandard(mg/l):

Calculation of the Cations:

${Me} = {\frac{{Me}\text{-}{value}\mspace{14mu} ( {{mg}\text{/}1} ) \times 100 \times {dilution}}{4 \times {weight}\mspace{14mu} {taken}\mspace{14mu} ( {{in}\mspace{14mu} g} ) \times {molar}\mspace{14mu} {mass}\mspace{14mu} ( {g\text{/}{mol}} )} = {{meq}\text{/}100\mspace{14mu} g}}$

Molar mass (g/mol): Ca=20.040; K=39.096; Li=6.94; Mg=12.156; Na=22.990;Al=8.994; Fe=18.616

In the case of so-called overactivated bentonites, i.e. those which wereactivated with an amount of, for example, sodium carbonate which isgreater than the stoichiometric amount, the sum of the amounts ofmonovalent cations determined may be greater than the CEC determined asstated above. In such cases, the total content of monovalent cations(Li, K, Na) is regarded as 100% of the CEC.

The invention is now illustrated in more detail with reference to thefollowing, non-limiting examples.

2. Determination of the BET Surface Area:

The determination was effected according to DIN 66131 (multipointmeasurement).

3. Determination of the Wet Sieve Residue:

With the use of pigments and fillers, it is of interest whether thematerial to be investigated contains coarse fractions which differ intheir particle size from the normal particles and how much of saidcoarse fractions said material contains. These fractions are determinedby sieving an aqueous suspension with water as wash liquid. The wetsieve residue is considered to be the residue determined under specifiedconditions.

Apparatuses: analytical balance, plastic beaker, Pendraulik LD 50;sieve: 200 mm diameter, mesh size 0.025 (25 μm), 0.045 mm (45 μm), 0.053mm (53 μm) or 0.063 mm (63 μm); ultrasonic bath.

First, a 5% strength suspension of the bentonite (oven dry, i.e. afterdrying at 110° C.) in 2000 g of water was prepared. For this purpose,the bentonite is stirred in at 930 rpm in about 5 min. After a stirringtime of a further 15 min at 1865 rpm, the suspension is poured into thecleaned and dried sieve (mesh size 45 μm) and washed with flowing tapwater while tapping until the wash water runs out clear. After thewashing of the sieve residue with tap water, the sieve is placed for 5min in an ultrasonic bath in order to sieve off the remaining finefractions. When using the sieve in the ultrasonic bath, it should beensured that no air remains between water surface and sieve bottom.After the ultrasonic treatment, rinse again briefly with tap water.Thereafter, the sieve is removed and the water in the ultrasonic bath isreplaced. The procedure in the ultrasonic bath is repeated untilcontamination of the water is no longer detectable. The sieve with theremaining residue is dried to constant weight (oven dry) in aforced-circulation drying oven. After cooling, the residue istransferred by means of a brush into a dish. Evaluation: wet sieveresidue (WSR) in (%), based on the amount weighed out.

4. Particle Size Determination According to Malvern:

This is a customary method. A Mastersizer from Malvern Instruments Ltd,UK, was used according to the manufacturer's instructions. Themeasurements were carried out with the sample chamber provided (“drypowder feeder”) in air and the values based on the sample volume weredetermined.

5. Investigation of Binding of Impurities: in the Investigation of theBinding of Impurities, the Following Procedure was Adopted:

a) Preparation of Paper Stock and Filtration:

The chosen paper stock (e.g. 45% of chemical pulp and 55% ofperoxide-bleached groundwood) can either be obtained directly from thepaper mill or stored in a refrigerator before use. The paper stock wasthen thoroughly shaken at 20 g absolutely dry and diluted to 2% withwarm demineralized water in a 2000 ml beaker. While being stirred at 400rpm, the paper stock batch warmed up to 40° C. with the aid of ahotplate. When the temperature is reached, the amount of adsorbent to betested is added to the paper stock batch with the aid of a Pasteurpipette. Thereafter, the adsorption time in the stock batch is fixed at30 min at 40° C. and the mixture is stirred for this time at 400 rpm.Thereafter, the paper stock batch of the adsorbent is diluted to 1%solids content with the aid of demineralized water (40° C.).

For the white water preparation, 1000 g of this dilute stock batch (1%by weight solids content) is drained in a drainage and retentionapparatus (Mütek DF3 03 from Mütek, Germany) for 420 seconds (170 μmsieve, stirring speed 700 rpm). The white water samples are investigatedanalytically.

b) Flow Cytometric Analysis of the White Water:

Here, so-called flow cytometry was used, as described in Vähäsalo etal., “Use of Flow Cytometry In Wet End Research”, Paper Technology, 44(1), page 45, February 2003 and additionally in “Effects Of pH andcalcium chloride on pitch in peroxide-bleached mechanical pulpsuspensions”, 7th European Workshop on Lignocellulosics and Pulp, Aug.26-29, 2002, Åbo/Finland. In brief, a light scattering method forcounting the particles is combined with fluorescence marking.

c) Gas Chromatographic Analysis of the White Water:

Here, the method of F. Orsa and B. Holmbom “A Convenient Method for theDetermination of Wood Extractives in Papermaking Process Waters andEffluents”, Journal of Pulp and Paper Science, Vol. 20 No. 12, December1994, pp J361, was used.

The following was found:

FIG. 1 shows a graph of the dependence of the concentration of theimpurity particles in the white water (filtrate water) on the type andamount of the adsorbent used (bentonite or talc).

The invention is now illustrated further with reference to thenon-limiting examples below.

EXAMPLE 1

The following materials were investigated for the binding of impurities.

1. Calcium Bentonite (Bentonite 1)

The analytical data of the calcium bentonite used are summarized intable 1. The proportions and the CEC are to be found in table 2.

TABLE 1 Analytical data of the calcium bentonite (bentonite 1) Watercontent 12.2% by weight pH (5% by weight of suspension 9.0 in water)Montmorillonite content 100% by weight (methylene blue method) Quartz0.5% by weight Calcite <1% by weight Specific surface area (BET) 86 m²/g

The wet sieve residue (45 μm) was less than 0.5% by weight.

TABLE 2 Proportion of the monovalent cations, based on the cationexchange capacity (CEC) of the calcium bentonite (bentonite 1); CEC(total) = 106 meg/100 g. Cation Proportion of the CEC (%) Na 34 K 2 Li 0

2. Cationized Talc (Product Malusil 75-7 K from Talc de Luzenac)

3. Bentonite According to the Invention (Bentonite 2)

Bentonite 2 was obtained from bentonite 1 by kneading bentonite 1 with5% by weight of sodium carbonate, based on the anhydrous bentonite,according to the above method, drying to a water content of 10% byweight and milling to a particle size corresponding to that of bentonite1 (comparison: table 2). By means of these processing steps, themineralogical data of the bentonite are not changed, so that themontmorillonite content and the content of impurity minerals remainunchanged. The BET surface area was 85±2 m²/g.

The analytical data for bentonite 2 are shown in table 3.

TABLE 3 Proportion of the monovalent cations, based on the cationexchange capacity (CEC) of the bentonite according to the invention(bentonite 2); CEC (total) = 102 meq/100 g of bentonite. CationProportion of the CEC (%) Na 98 K 2 Li 0

Using the two bentonites 1 and 2, the binding of impurities wasinvestigated as described in the method section. For carrying out thefiltration experiments, a paper stock which was taken from a papermachine and consisted of 45% of long fiber chemical pulp and 55% ofperoxide-bleached groundwood was used.

For comparison, in each case a “zero sample” was run, i.e. no adsorbentswere used for binding impurities.

For characterization of the filtrate waters (white waters) with regardto a reduction of impurities, the abovementioned flow cytometry wasused. The results are shown in FIG. 1. The amount of adsorbent used(bentonite or talc) is plotted against the concentration of the impurityparticles in the white water. It is clearly found that the bentonite 2according to the invention exhibits substantially better binding ofimpurities than bentonite 1 or talc even when a small amount of threekilograms per tonne, based on the paper pulp/suspension, is used in drypulp.

The content of fatty acids, lignins, sterols, steryl esters andtriglycerides was determined for the above samples by means of the gaschromatographic analysis (cf. method section). The bentonites 1 and 2were used in each case in an amount of 6 kg/t of paper (dry weight); thecationized talc was used in an amount of 11.25 kg/t of paper, since 6kg/t gives poor results. The values obtained are shown in table 4.

TABLE 4 Concentrations of individual impurities after the treatment withthe impurity-binding agents in mg/l according to gas chromatographyFatty Steryl Sample acids Lignins Sterols esters Triglycerides “Zero0.08 4.38 0.25 1.26 1.69 sample” Cat. talc 0.04 4.23 0.20 1.07 1.77Bentonite 1 0.05 4.16 0.07 0.46 0.96 Bentonite 2 0.04 4.07 0.06 0.310.67

As is evident in table 4, the sample treated with the bentonite 2according to the invention shows substantially better binding/removal offatty acids, lignins, styrenes, styryl esters and triglycerides bothcompared with the sample treated with cationized talc and with thesample treated with the calcium bentonite (bentonite 1) not according tothe invention.

In a further example, the bentonite according to the invention wascompared with conventional bentonites which have a proportion ofmonovalent cations, based on the CEC, of at least 0.7 (70%) but a CEC ofless than 85 meq/100 g.

Once again, substantially better binding of impurities by the bentoniteaccording to the invention was found in comparison with the conventionalbentonites, even when small amounts were used.

1. A method for binding impurities in paper production, comprising thefollowing steps: a) providing bentonite, with a proportion of monovalentcations of at least about 0.7 based on the cation exchange capacity(CEC) of the bentonite, wherein the CEC is greater than 85 meq/100 g; b)adding the bentonite to a paper pulp or fiber suspension; and c) bindingimpurities to the bentonite in the pulp or fiber suspension.
 2. Themethod as claimed in claim 1, characterized in that the proportion ofthe monovalent cations, based on the CEC of the bentonite, is more than0.8.
 3. The method as claimed in claim 1, characterized in that theproportion of calcium and/or magnesium ions in the bentonite, based onthe CEC of the bentonite, is less than 0.2.
 4. The method as claimed inclaim 1, characterized in that the particle size of the bentonite ischosen so that in the wet sieve residue less than 2% by weight, is 45μm.
 5. The method as claimed in claim 1, characterized in that themonovalent cations in the bentonite are selected from the groupconsisting of sodium, potassium, lithium and mixtures thereof.
 6. Themethod as claimed in claim 1, characterized in that the bentonite ispresent in particulate form having a median particle size (D50, based onvolume) of from 0.5 to 10 μm.
 7. The method as claimed in claim 1,characterized in that the addition of the bentonite is effected in theabsence of talc.
 8. The method as claimed in claim 1, characterized inthat the bentonite has a swellability of at least 25 ml/2 g.
 9. Themethod as claimed in claim 1, characterized in that the bentonite has aproportion of iron ions, based on the CEC, of less than about 0.005. 10.The method as claimed in claim 1, characterized in that the bentonitehas a BET surface area of less than 100 m²/g.
 11. The method as claimedin claim 1, characterized in that from about 0.5 to 10 kg of bentoniteare added per tonne of paper pulp or fiber suspension (dry weight). 12.The method as claimed in claim 1, characterized in that the paper pulpor fiber suspension contains a groundwood fraction.
 13. The method asclaimed in claim 12, characterized in that the groundwood fraction inthe paper pulp or the fiber suspension is at least 10% by weight, basedon the total pulp or fiber suspension (dry weight).
 14. The method asclaimed in claim 1, characterized in that no additional talc is added tothe paper pulp or fiber suspension.
 15. (canceled)
 16. (canceled) 17.(canceled)
 18. (canceled)
 19. The method as claimed in claim 1, whereinthe CEC of the bentonite is greater than 95 meq/100 g.
 20. The method asclaimed in claim 1 characterized in that the proportion of themonovalent cations, based on the CEC of the bentonite, is more than0.85.
 21. The method as claimed in claim 1 characterized in that theproportion of calcium and/or magnesium ions in the bentonite, based onthe CEC of the bentonite, is less than 0.15.
 22. The method as claimedin claim 1 characterized in that the particle size of the bentonite ischosen so that the wet sieve residue less than 2% by weight is 45 μm.23. The method as claimed in claim 1 characterized in that the bentoniteis present in particulate form having a medium particle size (D50, basedon volume) of from 2 to 6 μm.
 24. A method for binding hydrophobicimpurities in paper production, comprising the following steps: a)providing bentonite, with a proportion of monovalent cations of at leastabout 0.7 based on the cation exchange capacity (CEC) of the bentonite,wherein the CEC is greater than 85 meq/100 g; b) adding the bentonite toa paper, pulp or fiber suspension; and c) binding hydrophobic impuritiesto the bentonite in the pulp and fiber suspension.